Consequences of Mai 1960 major subduction earthquake in the Andes and on lacustrine and marine sedimentation of Northern Patagonia (Chile, Argentina).

EMMANUEL CHAPRON; D. ARIZTEGUI; SANDOR MULSOW; GUSTAVO VILLAROSA; MARIO PINO; VALERIA OUTES; CHARLET F.; ETIENNE JUVIGNIE

Zurich, Suiza

Congreso; 3rd Swiss Geoscience Meeting; 2005

University of Zurich and Swiss Federal Institute of Technology Zurich

In this part of Chile and Argentina located at 40-42°S, the active subduction setting of the Nazcaplate beneath the South America plate, the melting of the Patagonian Ice Sheet during the Lateglacial and the concomitant growth of large stratocones over the most active volcanoes of the Americas, resulted in a complex Late Quaternary geomorphology. Northern Patagonia is characterized by strong precipitations driven by the Westerlies over the SE Pacific and is including many sub-aqueous environments that are sensitive natural archives of past environmental changes: lakes of glacial, tectonic or volcanic origin, but also fjords and bays that were flooded by the postglacial sea level rise. Strong westerly winds and intense volcanic activity in the study area result in the formation of thick and unstable andosoils (soils developed on volcanic ash) draping the steep morphologies of the Andes. Such climate and soils favours the development of a very dense vegetation cover consisting of a temperate evergreen rain forest. In this study, the recent sedimentation processes in four contrasting lacustrine and marine basins of Northern Patagonia are documented by highresolution seismic reflection profiling (3.5 kHz) and short cores at selected sites in deep lacustrine basins. The regional correlation of the cores is provided by the combination of 137Cs dating in lakes Puyehue (Chile) and Frías (Argentina), and by the identification of the Cordon Caulle 1921-22 and 1960 tephras in lakes Puyehue and Nahuel Huapi (Argentina) and in their catchment areas. This event stratigraphy allows correlating across the Andes the formation of striking sedimentary events in these basins with the consequences of May-June 1960 earthquakes and the well-documented induced Cordon Caulle eruption next to the Puyehue volcano; only 38 hours after the main shock offshore Valdivia. This rhyodacitic fissure eruption was triggered along the Liquiñe-Ofqui Fault Zone (LOFZ), a major active intra-arc shear zone in this part of the Andes. Thousands of landslides were also triggered in the Andes along the LOFZ, during the multiple seismic shocks of the major 21-22 Mai 1960 subduction earthquakes (Mw 9.5). Some of these well-documented landslides were catastrophic and deeply affected the drainage basins of most of the glacial lakes from the Chilean Lake District. In Argentina earthquake-triggered landslides were also documented, but the most striking historical chronicles are the descriptions of violent lake water oscillations or destructive waves triggered by the main seismic shock, in some of the largest Argentinean glacial lakes between 39.5 and 46°S. Our study shows that while this catastrophe induced a major hyperpycnal flood deposit of ca. 3.106 m3 in the proximal basin of Lago Puyehue, it only triggered and unusual organic rich layer in the proximal basin of Lago Frías, but destructive waves and a large sub aqueous slide in the distal basin of Lago Nahuel Huapi. A very recent megaturbidite in the two distal basins of Reloncavi fjord (Chile) located close to the LOFZ is also suggesting that 1960 co-seismic movements in this area may have triggered the remobilization of ca. 187.106 m3 of marine sediments. Sub bottom profiling in these contrasting lacustrine and marine basins at both sides of the Andes are highlighting the recurrent incidence of major sedimentary events during the Late Holocene. In order to confirm these reconstructions on the impact of 1960 earthquakes in Northern Patagonia, further studies should include dense grids of highresolution seismic profiling and detailed studies of sediment cores. Moreover, these deep lacutrine and marine basins have the potential to document the recurrence of major subduction earthquakes in this part of South America over several millennia137Cs dating in lakes Puyehue (Chile) and Frías (Argentina), and by the identification of the Cordon Caulle 1921-22 and 1960 tephras in lakes Puyehue and Nahuel Huapi (Argentina) and in their catchment areas. This event stratigraphy allows correlating across the Andes the formation of striking sedimentary events in these basins with the consequences of May-June 1960 earthquakes and the well-documented induced Cordon Caulle eruption next to the Puyehue volcano; only 38 hours after the main shock offshore Valdivia. This rhyodacitic fissure eruption was triggered along the Liquiñe-Ofqui Fault Zone (LOFZ), a major active intra-arc shear zone in this part of the Andes. Thousands of landslides were also triggered in the Andes along the LOFZ, during the multiple seismic shocks of the major 21-22 Mai 1960 subduction earthquakes (Mw 9.5). Some of these well-documented landslides were catastrophic and deeply affected the drainage basins of most of the glacial lakes from the Chilean Lake District. In Argentina earthquake-triggered landslides were also documented, but the most striking historical chronicles are the descriptions of violent lake water oscillations or destructive waves triggered by the main seismic shock, in some of the largest Argentinean glacial lakes between 39.5 and 46°S. Our study shows that while this catastrophe induced a major hyperpycnal flood deposit of ca. 3.106 m3 in the proximal basin of Lago Puyehue, it only triggered and unusual organic rich layer in the proximal basin of Lago Frías, but destructive waves and a large sub aqueous slide in the distal basin of Lago Nahuel Huapi. A very recent megaturbidite in the two distal basins of Reloncavi fjord (Chile) located close to the LOFZ is also suggesting that 1960 co-seismic movements in this area may have triggered the remobilization of ca. 187.106 m3 of marine sediments. Sub bottom profiling in these contrasting lacustrine and marine basins at both sides of the Andes are highlighting the recurrent incidence of major sedimentary events during the Late Holocene. In order to confirm these reconstructions on the impact of 1960 earthquakes in Northern Patagonia, further studies should include dense grids of highresolution seismic profiling and detailed studies of sediment cores. Moreover, these deep lacutrine and marine basins have the potential to document the recurrence of major subduction earthquakes in this part of South America over several millennia6 m3 in the proximal basin of Lago Puyehue, it only triggered and unusual organic rich layer in the proximal basin of Lago Frías, but destructive waves and a large sub aqueous slide in the distal basin of Lago Nahuel Huapi. A very recent megaturbidite in the two distal basins of Reloncavi fjord (Chile) located close to the LOFZ is also suggesting that 1960 co-seismic movements in this area may have triggered the remobilization of ca. 187.106 m3 of marine sediments. Sub bottom profiling in these contrasting lacustrine and marine basins at both sides of the Andes are highlighting the recurrent incidence of major sedimentary events during the Late Holocene. In order to confirm these reconstructions on the impact of 1960 earthquakes in Northern Patagonia, further studies should include dense grids of highresolution seismic profiling and detailed studies of sediment cores. Moreover, these deep lacutrine and marine basins have the potential to document the recurrence of major subduction earthquakes in this part of South America over several millennia